I would always ask for DNA testing, even if I was married to the woman.
Now in Texas, you can be ordered to pay child support the rest of a child's life -- if a child is disabled!
Therefore, it's worth a few moment's of potential awkwardness or possible anger to ask for a DNA test!
Be sure it's your kid!
Once you are on the birth certificate, it's difficult to get your name off. Yes, it can be done but it's much easier to just never have your name on it in the first place!